Activated carbon is a microcrystalline, nongraphitic form of carbon which has been processed to increase internal porosity. Activated carbons are characterized by a large specific total surface area (typically in the range of 500-2500 m.sup.2 /g). Such large surface areas make activated carbons useful in a number of applications, including electrical energy storage devices.
The available surface area of activated carbon is dependent on its pore volume. Since the surface area per unit volume decreases as individual pore size increases, large surface area generally is maximized by maximizing the number of pores of very small dimensions and/or minimizing the number of pores of very large dimensions. Pore sizes are defined herein as micropores (pore width&lt;2.0 nanometers or nm), mesopores (pore width=2.0 to 50.0 nm), and macropores (pore width&gt;50.0 nm).
This invention relates to carbons derived from lignocellulosic materials (particularly wood-based lignocellulosic materials) which are suitable for use in electrical energy storage devices of the general type disclosed in U.S. Pat. Nos. 3,536,963, 3,634,736, 3,648,126, 4,542,444, and 5,144,537 (which are hereby incorporated by reference). These devices, which are commonly known as carbon double layer capacitors or CDLCs, are usually comprised of a pair of electrodes (at least one of which is a carbon paste electrode), a separator, and an ionic-insulating, electric current-conducting collector.
The carbon paste electrode in a CDLC is generally manufactured by mixing finely divided petroleum-derived carbon particles with an electrolyte to form a paste, and then subsequently forming an electrode from the paste. While the use of activated petroleum-derived carbons in CDLCs is well-known, the practical use of activated wood-based carbons in CDLCs has, until now, only been the subject of theoretical speculation.
The use of carbon double layer capacitors in various applications is well established. Such capacitors can be described in terms of their energy density (kilowatt hour/kg) and power density (watts/kg) characteristics. High energy density capacitors store a relatively high capacitance which is discharged slowly over a period of minutes. In contrast, high power density capacitors can deliver their capacitance or energy quickly (in a few milliseconds). Various practical applications have different requirements in terms of energy and power. For example, memory back-up devices require a reasonably high energy density, but do not require that the energy be released quickly (low power, long discharge time). On the other hand, an application such as starting an automobile engine requires very high power and much of the energy must be released in a few milliseconds. Other applications require combinations of energy and power densities intermediate to these two extremes.
While activated carbons having high surface areas are employed in conventional carbon double layer capacitors, it has not been recognized that certain types of activated carbons have a large and demonstrable effect upon the energy and deliverable power densities of the devices. It has now been discovered that utilizing carbons with high mesoporosity in a CDLC can significantly increase the power density.
It is, therefore, an object of this invention to provide a method for producing improved carbons derived from lignocellulosic material suitable for use in carbon double layer capacitors.
It is a further object of this invention to provide an improved carbon derived from lignocellulosic material for use in high power density CDLCs.
It is yet another object of this invention to provide an improved carbon derived from lignocellulosic material which exhibit a relatively specific pore volume and mean pore radius.